This invention relates generally to the speed control of electric motors, and specifically to improvements in a speed control system for motors, particularly those suitable for use in a disk drive, such for example as a CD-ROM drive, for moving the transducer or pickup assembly across the track turns of the rotating data storage disk, among other applications.
Brushless d.c. motors have found widespread use for rotating a data storage disk or for moving a transducer across the data tracks on the disk. Fine control of motor speed is an absolute requirement in such applications. U.S. Pat. No. 5,371,635 to Sakaguchi et al., assigned to the assignee of the instant application, is hereby cited as teaching a system for generating pulses indicative of the angular position and rotational speed of a data storage disk. This prior art system is unsatisfactory for its complexity and expensiveness of construction as an inevitable result of the use of a motor speed sensor aside from the control electronics.
Japanese Unexamined Patent Publication No. 3-16066 suggests, for speed control of a brushless d.c. motor, to utilize the Hall-effect devices, the magnetoelectric converters, that have been customarily built into this type of motor for detection of the angular position of its rotor relative to the stator. The rotor position signals produced by the magnetoelectric converters are differentiated and further processed into a motor speed signal indicative of the actual rotational speed of the motor preparatory to motor speed control.
This second prior art system gains a definite advantage over the first recited one in that it does without any dedicated motor speed sensor. It is still objectionable, however, first because the control electronics, particularly those for production of the motor speed signal from the rotor position signals supplied by the magnetoelectric converters, is not as simple and inexpensive as can be desired. Secondly, the motor speed signal so produced is inconveniently high in ripple percentage, running counter to the objective of fine motor speed control.